That 4-pin plug supplies power to the processor. Without it, the system won't boot up. Lights or fans are no indicator that the PSU is good. All that proves is that the PSU can put out enough power for the lights or fans, it doesn't necessarily mean it can power up the entire system. Try unplugging everything from the PSU except the two main plugs to the board. Remove all cards (switch monitor cable to unboard video port, if available) & unplug all USB devices. Then see if the board will fire up.

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